Tumor Transplantation for Assessing the Dynamics of Tumor-Infiltrating CD8<sup>+</sup> T Cells in Mice

T cell-mediated immunity plays a crucial role in immune responses against tumors, with cytotoxic T lymphocytes (CTLs) playing the leading role in ...

Third Military Medical University

Follicular regulatory T (Tfr) cells differentiate from conventional regulatory T (Treg) cells and suppress excessive germinal center (GC) responses by acting on both GC B cells and T follicular helper (Tfh) cells. Here, we examined the impact of mTOR, a serine/threonine protein kinase that senses and integrates diverse environmental cues, on the differentiation and functional competency of Tfr cells in response to protein immunization or viral infection. By genetically deleting Rptor or Rictor, essential components for mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), respectively, we found that mTORC1 but not mTORC2 is essential for Tfr differentiation. Mechanistically, mTORC1-mediated phosphorylation of the transcription factor STAT3 induced the expression of the transcription factor TCF-1 by promoting STAT3 binding to the Tcf7 5'-regulatory region. Subsequently, TCF-1 bound to the Bcl6 promoter to induce Bcl6 expression, which launched the Tfr cell differentiation program. Thus, mTORC1 initiates Tfr cell differentiation by activating the TCF-1-Bcl-6 axis during immunization or infection.

The Kinase mTORC1 Promotes the Generation and Suppressive Function of Follicular Regulatory T Cells.

B lymphocytes and their differentiated daughter cells are charged with responding to invading pathogens and producing protective antibodies against these pathogens. The physiology of B cells is intimately connected with the function of the B cell antigen receptor (BCR). Upon activation of BCR, transmembrane signals are generated, and several downstream pathways are activated, which provide a primary directive for the cell's subsequent response. mTOR is a serine/threonine kinase that controls cell proliferation and metabolism in response to a diverse range of extracellular stimuli. The activation of mTOR signaling downstream of PI3K/Akt activity by B cell receptor (BCR) engagement has been generally assumed to be essential for B cell responses. This chapter seeks to present two protocols to evaluate mTOR activity in B cells bearing BCR specific to 4-hydroxy-3-nitrophenylacetyl (NP)-hapten.

Flow Cytometry Analysis of mTOR Signaling in Antigen-Specific B Cells.

The long-term persistence of viral antigens drives virus-specific CD8 T cell exhaustion during chronic viral infection. Yet exhausted, CD8 T cells are still endowed with certain levels of effector function, by which they can keep viral replication in check in chronic infection. However, the regulatory factors involved in regulating the effector function of exhausted CD8 T cell are largely unknown. Using mouse model of chronic LCMV infection, we found that the deletion of transcription factor TCF-1 in LCMV-specific exhausted CD8 T cells led to the profound reduction in cytokine production and degranulation. Conversely, ectopic expression of TCF-1 or using agonist to activate TCF-1 activities promotes the effector function of exhausted CD8 T cells. Mechanistically, TCF-1 fuels the functionalities of exhausted CD8 T cells by promoting the expression of an array of key effector function-associated transcription regulators, including Foxo1, Zeb2, Id3, and Eomes. These results collectively indicate that targeting TCF-1 mediated transcriptional pathway may represent a promising immunotherapy strategy against chronic viral infections by reinvigorating the effector function of exhausted virus-specific CD8 T cells.

The Transcription Factor TCF1 Preserves the Effector Function of Exhausted CD8 T Cells During Chronic Viral Infection.

T follicular helper (T) cells are crucial for effective humoral immunity by providing the required signals to cognate B cells and promoting germinal center (GC) formation. Many intrinsic and extrinsic factors have been reported to be involved in the multistage, multifactorial differentiation process of T cells. By comparing gene expression between T cells and T1 cells based on published GEO data, we found selective and high expression of sclerostin domain-containing protein 1 (SOSTDC1) in T cells but not in T1 cells; however, it is unclear whether SOSTDC1 is important for the differentiation and/or function of T cells. Using a mouse model of acute lymphocytic choriomeningitis virus (LCMV) infection, we confirmed the selective expression of SOSTDC1 in T cells compared to that in T1 cells, but the ablation of SOSTDC1 did not affect T cell differentiation or effector function. Thus, our results indicate that the SOSTDC1 protein is merely a specific marker of T cells but does not play a functional role in the differentiation of T cells during acute viral infection.

Sclerostin domain-containing protein 1 is dispensable for the differentiation of follicular helper and follicular regulatory T cells during acute viral infection.

During tumorigenesis, tumor infiltrating regulatory T (Treg) cells restrict the function of effector T cells in tumor microenvironment and thereby promoting tumor growth. The anti-tumor activity of effector T cells can be therapeutically unleashed, and is now being exploited for the treatment of various types of human cancers. However, the immune suppressive function of Treg cells remains a major hurdle to broader effectiveness of tumor immunotherapy. In this article, we reported that the deletion of Bcl6 specifically in Treg cells led to stunted tumor growth, which was caused by impaired Treg cell responses. Notably, Bcl6 is essential in maintaining the lineage stability of Treg cells in tumor microenvironment. Meanwhile, we found that the absence of follicular regulatory T (Tfr) cells, which is a result of Bcl6 deletion in Foxp3 cells, was dispensable for tumor control. Importantly, the increased Bcl6 expression in Treg cells is associated with poor prognosis of human colorectal cancer and lymph node metastasis of skin melanoma. Furthermore, Bcl6 deletion in Treg cells exhibits synergistic effects with immune checkpoint blockade therapy. Collectively, these results indicate that Bcl6 actively participates in regulating Treg cell immune responses during tumorigenesis and can be exploited as a therapeutic target of anti-tumor immunity.

Yiding Li

Shigatse Branch,
Xinqiao Hospital,
Shigatse Branch, Xinqiao Hospital

Third Military Medical University

Tumor Transplantation for Assessing the Dynamics of Tumor-Infiltrating CD8⁺ T Cells in Mice

Yiding Li

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Third Military Medical University

Tumor Transplantation for Assessing the Dynamics of Tumor-Infiltrating CD8+ T Cells in Mice

Shigatse Branch,
Xinqiao Hospital,
Shigatse Branch, Xinqiao Hospital

Tumor Transplantation for Assessing the Dynamics of Tumor-Infiltrating CD8⁺ T Cells in Mice